Composite structures are used in a wide variety of applications. For example, in aircraft construction, composites are used in increasing quantities to form the fuselage, wings, tail section and other aircraft components. The fabrication of composite parts generally entails laying up composite material on a tool or die and curing the composite material at elevated temperatures and pressures to form a rigid composite structure. Unfortunately, conventional practices for laying up and curing the composite material typically include numerous additional steps in order to produce a final composite structure having the desired mechanical properties.
For example, prior to or during curing of the composite layup, it may be necessary to compress or debulk and/or consolidate the composite layup in order to prevent the occurrence of voids in the cured composite structure and to reduce the thickness of the composite layup such that the cured composite structure meets dimensional design requirements. The reduced thickness of the composite layup as a result of consolidation may also be necessary to achieve the desired fiber volume fraction of the cured composite structure. In this regard, consolidation may reduce the overall volume and/or weight of the resin in the composite structure relative to the volume or weight of the load-carrying fibers that make up the composite material.
In one prior art curing or consolidating method, the composite layup may be vacuum bagged or sealed to the tool. A vacuum may be applied to the vacuum bag in order to promote the dispersion of uncured resin throughout the composite layup and to draw out air and volatiles (i.e., curing byproducts) from the composite layup that may otherwise become trapped in the composite layup during curing.
In order to generate a sufficient amount of external pressure on the composite layup, conventional practices include vacuum bagging of the composite layup assembly and transferring the composite layup to an autoclave. The pressure and temperature within the autoclave are then increased until the composite layup reaches the curing pressure and temperature. The composite layup must typically be held at the curing temperature for a predetermined period of time while external pressure is applied to an exterior of the vacuum bag and vacuum pressure is applied to an interior of the vacuum bag. The curing cycle may further require a stepwise or gradual increase and/or reduction in the temperature of the composite layup while external pressure and vacuum pressure is maintained.
Following curing, the pressure and temperature of the cured composite structure must be reduced to allow for removal of the bagged composite layup assembly from the autoclave followed by removal of the cured composite structure from the vacuum bag and tool. As may be appreciated, autoclaves of sufficiently large size for handling correspondingly large composite layups represent a significant capital equipment expenditure which adds to the overall cost and complexity of fabricating a composite structure. Furthermore, the amount of time required to prepare a composite layup for autoclave operations and to complete a curing or consolidating process using the autoclave represents a significant portion of the total fabrication cycle time for a composite structure. In this regard, because autoclaves typically rely on convective heating to elevate the temperature of the composite layup, the relatively large thermal mass of the tool as compared to the thermal mass of the composite layup results in an extended period of time for reaching the curing temperature and then cooling the composite layup and tool.
Attempts at reducing reliance on autoclaves to provide the requisite consolidation temperatures and pressures include the use of hydraulic presses. Although generally satisfactory for forming composite structures of relatively small size, hydraulic presses may be limited in the amount of pressure that can be developed. As a result, the use of hydraulic presses for consolidating and curing large composite layups is limited. Furthermore, curing cycles for certain composite layups may require the application of precise levels of temperature and pressure which may be unachievable using conventional hydraulic presses. In addition, hydraulic presses may present cleanliness and maintenance challenges in consideration of the use of hydraulic oil as the working fluid and the various components such as accumulators, pumps, sealing mechanisms and other hardware typically associated with hydraulic presses.
As can be seen, there exists a need in the art for a system and method for curing a composite layup that eliminates the need for an autoclave. In this regard, there exists a need in the art for a system and method for curing a composite layup that can rapidly achieve the requisite temperatures and pressures required for curing. In addition, there exists a need in the art for a system and method for curing a composite layup that allows for application of the curing pressures required to form composite layups of relatively large size. Finally, there exists a need in the art for a low-cost system and method for curing composite layups that can be performed in a reduced amount of time.